1. Field of the Invention
The present invention relates to a machine tool for machining workpieces, comprising a workpiece support for mounting the workpiece, a tool holder, which is adapted to be moved at least along one axis, for holding a tool for machining the workpiece, and workpiece holding units for fixing and centering the workpiece during the machining.
The invention further relates to a corresponding method for machining workpieces, comprising the steps of: mounting the workpiece on a workpiece support, holding a tool by a tool holder, which is adapted to be moved at least along one axis, machining the workpiece with the tool, and fixing and centering the workpiece during machining by workpiece holding units.
Further, the present invention also relates to a workpiece holding unit for fixing and centering a workpiece during the machining by means of an above mentioned machine tool.
2. Related Prior Art
Machine tools of the type stated at the outset are used, for example, for machining of automobile wheel rims, in particular for applying drilled holes for subsequent valve lead-through and for the wheel bolts. For the purpose of fixing a rim during machining, in the case of a known machine tool the rim is placed on the workpiece support, e.g., the workpiece table or a cradle plate of a cradle, in such a way that, for the purpose of centering the rim, the hub hole is placed onto an arbor provided on the workpiece support and the lower rim flange rests on the workpiece support. Further, for the purpose of fixing the rim, a plurality of piston cylinder units, for example three, are provided on the workpiece support. These piston cylinder units are arranged in a distributed manner around the rim, and each carry a hold-down device as a holding element.
For the purpose of fixing the rim, in the case of this machine tool, the positions of the piston cylinder units must first be matched to the diameter of the rim to be machined, for which purpose the piston cylinder units have to be moved along a travel rail on the workpiece support and then anchored. Then, by means of a swiveling arm, the hold-down device is in each case swiveled to the rim and then folded down from above onto the lower rim flange, whereby the rim is pressed against the workpiece support from above.
Apart from the fact that, in the case of the known machine tool, the piston cylinder units having the hold-down devices that can be swiveled and folded down have a relatively complex mechanical system, there is also the disadvantage that these piston cylinder units are directly exposed to the stock cuttings produced during machining and to the coolant, and consequently rapidly accumulate dirt or even become worn. Further, the piston cylinder units occupy a lot of space in the working area, such that only rims up to a maximum diameter can be machined, since otherwise the robot arm usually employed for loading the rims into and out of the working area would collide with the piston cylinder units. Furthermore, the positions of the piston cylinder units have to be reset if changes are made to the diameter of the rims to be machined, and a plurality of motions, of differing axes, are required in order to fix the rim. Moreover, a centering arbor is essential for centering.